The present invention relates to oxide superconducting tunnel junctions and manufacturing method for the same.
Josephson devices are superconducting electric devices which have been actually used in the field such as in the form of SQUID. The device operates based on the Josephson effect which takes place along a tunnel junction. On the other hand, superconducting oxide ceramics have attracted the interests of scientists as promising the future superconducting device such as switching devices for supercomputor which can operate at liquid nitrogen temperature.
In order to form a Josephson junction, a barrier film has to be interposed between superconducting regions. The thickness of the junction is on the order of the coherent length of superconducting carriers. While the coherent lengths in metal superconductors are of the order of several hundreds of angstroms, the coherent lengths in oxide ceramic superconductors are of the order of several tens of angstroms. It means that the thickness of the barrier film have to be 100 .ANG. or less.
The difficulties in forming Josephson junction is attributed partially to the fact that ceramic superconducting crystals are hardly deposited on a barrier film such as MgO or Al.sub.2 O.sub.3, which usually has a different crystalline structure than the superconducting ceramic. Namely, when forming a lower superconducting film, a barrier film and an upper superconducting film stacked in this order, the superconductivity of the upper superconducting film is degraded because of its poor crystallinity ensuring from the influence of the underlying barrier film.